Optimization of a new interatomic potential to investigate the thermodynamic properties of hypo-stoichiometric mixed oxide fuel U1-y Pu y O2-x.

The behaviour of stoichiometric U1-y Pu y O2 compounds used as nuclear fuel is relatively well understood. Conversely, the effects of stoichiometry deviation on fuel performance and fuel stability are intricate and poorly studied. In order to investigate what affect these have on the thermophysical properties of hypo-stoichiometric U1-y Pu y O2-x mixed oxide fuel, new interaction parameters based on the many-body CRG (Cooper-Rushton-Grimes) potential formalism were optimized. The new potential has been fitted to match experimental lattice parameters of U0.70Pu0.30O1.99 (O/M = 1.99) and U0.70Pu0.30O1.97 (O/M = 1.97), where M represents the total amount of metallic cations, through a rigorous procedure combining classical molecular dynamic and classical molecular Monte Carlo simulation methods. This new potential provides an excellent description of the U1-y Pu y O2-x system. Concerning lattice parameter, although fitted on only one Pu content (30%) and two stoichiometries (1.99 and 1.97), our potential allows good predictions compared to available experimental results as well as to available recommendations in wide ranges of O/M ratio, Pu content and temperature. For the U0.70Pu0.30O2-x hypo-stoichiometric system (30% Pu content and O/M ratio ranging from 1.94 to 2.00), some direct properties (lattice parameter and enthalpy) and some derivative properties (linear thermal expansion coefficient and specific heat) were systematically investigated from room temperature up to the expected melting temperatures and a good agreement with experiments is found. Moreover, our potential shows good transferability to the plutonium sesquioxide Pu2O3 system.

Classical force field for hydrofluorocarbon molecular simulations. Application to the study of gas solubility in poly(vinylidene fluoride).

A classical all-atoms force field for molecular simulations of hydrofluorocarbons (HFCs) has been developed. Lennard-Jones force centers plus point charges are used to represent dispersion-repulsion and electrostatic interactions. Parametrization of this force field has been performed iteratively using three target properties of pentafluorobutane: the quantum energy of an isolated molecule, the dielectric constant in the liquid phase, and the compressed liquid density. The accuracy and transferability of this new force field has been demonstrated through the simulation of different thermophysical properties of several fluorinated compounds, showing significant improvements compared to existing models. This new force field has been applied to study solubilities of several gases in poly(vinylidene fluoride) (PVDF) above the melting temperature of this polymer. The solubility of CH4, CO2, H2S, H2, N2, O2, and H2O at infinite dilution has been computed using test particle insertions in the course of a NpT hybrid Monte Carlo simulation. For CH4, CO2, and their mixtures, some calculations beyond the Henry regime have also been performed using hybrid Monte Carlo simulations in the osmotic ensemble, allowing both swelling and solubility determination. An ideal mixing behavior is observed, with identical solubility coefficients in the mixtures and in pure gas systems.

Molecular modeling of the liquid-vapor interfaces of a multi-component mixture: Prediction of the coexisting densities and surface tensions at different pressures and gas compositions.

Two-phase molecular simulations are performed in order to report the interfacial tensions and the coexisting densities of a multicomponent mixture (nitrogen + methane) + water for five gas compositions in the pressure range of 1-30 MPa at 298 K. The interfacial tensions are calculated using different definitions and the long range corrections of the surface tensions are considered using expressions designed for multicomponent mixtures and each definitions. We can conclude that the agreement with experiments is quantitative with deviations smaller than 5% for the interfacial tensions and 2% for the densities. The interfacial region is described in terms of specific arrangements of the gas components at the water surface.

Monte Carlo calculation of the methane-water interfacial tension at high pressures.

Monte Carlo simulations have been performed in the Np(N)AT statistical ensemble to study the methane-water mixture as a function of pressure. The interfacial tensions are calculated with different definitions and are reported for pressures from 1 to 50 MPa. The interfacial tensions, coexisting densities, and composition of the methane and water phases are shown to be in good agreement with the corresponding experimental properties. The interfacial region has been described through the profiles of the number of hydrogen bonds, the coordination number of each species, and the different energy contributions. We complete this study by a theoretical investigation of the thermal and mechanical equilibria in the binary mixture. We have also examined the profile of the intrinsic and long range correction parts of the interfacial tension along the normal to the water surface.

Monte Carlo simulations of the pressure dependence of the water-acid gas interfacial tensions.

We report two-phase Monte Carlo (MC) simulations of the binary water-acid gas mixtures at high temperature and high pressure. Simulations are performed in the Np(N)AT ensemble in order to reproduce the pressure dependence of the interfacial tensions of the water-CO(2) and water-H(2)S mixtures. The interfacial tension of the binary water-CO(2) mixture is determined from 5 to 45 MPa along the isotherm T = 383 K. Water-H(2)S interfacial tensions are computed along one supercritical isotherm (T = 393 K) in a pressure range of 1-15 MPa. The temperature and pressure conditions investigated here by the MC simulations are typical of the geological storage conditions of these acid gases. The coexisting densities and the compositions of the water-rich and acid-gas-rich phases are compared with experiments and with data calculated from Gibbs ensemble Monte Carlo (GEMC) simulations.

Calculation of the surface tension of cyclic and aromatic hydrocarbons from Monte Carlo simulations using an anisotropic united atom model (AUA).

We report the calculation of the surface tension of cycloalkanes and aromatics by direct two-phase MC simulations using an anisotropic united atom model (AUA). In the case of aromatics, the polar version of the AUA-4 (AUA 9-sites) model is used. A comparison with the nonpolar models is carried out on the surface tension of benzene. The surface tension is calculated from different routes: the mechanical route using the Irving and Kirkwood (IK) and Kirkwood-Buff (KB) expressions; the thermodynamic route by using the test-area (TA) method. The different operational expressions of these definitions are presented with those of their long range corrections. The AUA potential allows to reproduce very well the dependence of the surface tension with respect to the temperature for cyclopentane, cyclohexane, benzene and toluene.

Calculation of the surface tension from Monte Carlo simulations: does the model impact on the finite-size effects?

We report two-phase Monte Carlo simulations of the liquid-vapor interface of the Lennard-Jones (LJ) fluids in order to study the impact of the methodology used for the energy calculation on the oscillatory behavior of the surface tension with the system sizes. The surface tension values are illustrated through the LJ parameters of methane. The first methodology uses a standard truncated LJ potential, the second one adds a long range correction (LRC) contribution to the energy into the Metropolis scheme, and the third one uses a LJ potential modified by a polynomial function in order to remove the discontinuities at the cutoff distance. The surface tension is calculated from the mechanical and thermodynamic routes and the LRCs to the surface tension are systematically calculated from appropriate expressions within these definitions. The oscillatory behavior has been studied as a function of the size of the interfacial area and of the length of the dimension perpendicular to the surface. We show that the methodology has an important effect on the oscillatory variation in the surface tension with the system size. This oscillatory variation in the surface tension with the system size is investigated through its intrinsic and LRC contributions. We complete this work by studying the dependence of the surface tension with respect to the cutoff distance when the LRC part to the energy is considered into the Metropolis scheme.

Surface tensions of linear and branched alkanes from Monte Carlo simulations using the anisotropic united atom model.

The anisotropic united atoms (AUA4) model has been used for linear and branched alkanes to predict the surface tension as a function of temperature by Monte Carlo simulations. Simulations are carried out for n-alkanes ( n-C5, n-C6, n-C7, and n-C10) and for two branched C7 isomers (2,3-dimethylpentane and 2,4-dimethylpentane). Different operational expressions of the surface tension using both the thermodynamic and the mechanical definitions have been applied. The simulated surface tensions with the AUA4 model are found to be consistent within both definitions and in good agreement with experiments.

Expressions for local contributions to the surface tension from the virial route.

The expression of the surface tension using the virial route has been reinvestigated in order to establish a local version of the surface tension and of its long-range corrections. In fact, giving a local surface tension is very important for the simulation from a methodological viewpoint. It is also of basic interest to associate the profile of the intrinsic part of the surface tension with that of the long-range corrections to make the surface tension calculation consistent between the different approaches that can be used. Working expressions for two-phase systems interacting through dispersion-repulsion (Lennard-Jones) and Coulombic (Ewald summation) interactions are proposed. Different operational expressions of the surface tension are compared in the cases of n -pentane, carbon dioxide, and water liquid-vapor equilibria for which the orders of magnitude between the electrostatic and dispersion forces are different.

Surface tension of water and acid gases from Monte Carlo simulations.

We report direct Monte Carlo (MC) simulations on the liquid-vapor interfaces of pure water, carbon dioxide, and hydrogen sulfide. In the case of water, the recent TIP4P/2005 potential model used with the MC method is shown to reproduce the experimental surface tension and to accurately describe the coexistence curves. The agreement with experiments is also excellent for CO(2) and H(2)S with standard nonpolarizable models. The surface tensions are calculated by using the mechanical and the thermodynamic definitions via profiles along the direction normal to the surface. We also discuss the different contributions to the surface tension due to the repulsion-dispersion and electrostatic interactions. The different profiles of these contributions are proposed in the case of water.

Multiple histogram reweighting method for the surface tension calculation.

The multiple histogram reweighting method takes advantage of calculating ensemble averages over a range of thermodynamic conditions without performing a molecular simulation at each thermodynamic point. We show that this method can easily be extended to the calculation of the surface tension. We develop a new methodology called multiple histogram reweighting with slab decomposition based on the decomposition of the system into slabs along the direction normal to the interface. The surface tension is then calculated from local values of the chemical potential and of the configurational energy using Monte Carlo (MC) simulations. We show that this methodology gives surface tension values in excellent agreement with experiments and with standard NVT MC simulations in the case of the liquid-vapor interface of carbon dioxide.